The Solar System Portal
The Solar System consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their moons, five currently identified dwarf planets and their seven known moons, and billions of small bodies. This last category includes asteroids, Kuiper belt objects, comets, meteoroids and interplanetary dust. In broad terms, the charted regions of the Solar System consist of the Sun, four terrestrial inner planets, an asteroid belt composed of small rocky bodies, four gas giant outer planets, and a second belt, called the Kuiper belt, composed of icy objects. Beyond the Kuiper belt lies the scattered disc, the heliopause, and ultimately the hypothetical Oort cloud. In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Six of the eight planets are in turn orbited by natural satellites, usually termed "moons" after Earth's Moon, and each of the outer planets is encircled by planetary rings of dust and other particles. All the planets except Earth are named after gods and goddesses from Greco-Roman mythology. The five dwarf planets are Pluto, Makemake, and Haumea, the three largest known Kuiper belt objects; Ceres, the largest object in the asteroid belt; and Eris, the largest known object in the scattered disc.
Io (pronounced /ˈaɪoʊ/ eye'-oe, or as Greek Ῑώ) is the innermost of the four Galilean moons
and, with a diameter of 3,642 kilometers, the fourth largest moon in the Solar System
. It was discovered in 1610 by Galileo Galilei
, along with the other Galilean satellites. This discovery furthered the adoption of the Copernican model
of the Solar System and the development of Kepler's laws of motion. Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide
core. Io has one of the most geologically active surfaces in the solar system, with over 400 active volcanoes. This extreme geologic activity is the result of tidal heating
from friction generated within Io's interior by Jupiter's varying pull. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (310 mi). Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's Mount Everest
. Most of Io's surface is characterized by extensive plains coated with sulfur
and sulfur dioxide
The Moon is the only natural satellite of Earth. During the ancient period, it was not uncommon for cultures to believe that the Moon died each night, thus descending into the underworld. As late as the 1920s (or so), it was believed that the Moon might have a breathable atmosphere. In 1969, Neil Armstrong and Buzz Aldrin became the first humans to land on the Moon.
Did you know...
- ...that the passing of the Great Comet of 1577 (pictured) caused almost century-long debate, during which Galileo argued that comets were merely optical illusions?
- ...that there is no widely accepted explanation for geographic features called Carolina bays, but that meteors may be the cause?
- ...that the Oort cloud, a postulated spherical cloud of comets around the Sun, is thought to be the origin of comets in the Solar System?
In the news
Things you can do